The present invention relates to a blood pumping system having a particular manner of generating a backflow warning to warn of blood backflow through a non-occlusive blood pump.
Blood pumping systems are used in clinical situations, such as open-heart surgery, to provide blood circulation for the patient undergoing the surgery. A typical blood pumping system draws blood from a patient's venous system into an external conduit connected to the inlet of a blood pump. The blood is pumped into to the patient's arterial system via a second conduit connected to the outlet of the blood pump. The first and/or second conduit may be connected to other devices to provide blood filtering, heating, oxygenation, collection, etc.
While various types of blood pumps have been used in blood pumping systems, one particular type of pump that has been found to be generally advantageous is a centrifugal pump. Such a pump is described in U.S. Pat. No. 4,589,822 to Clausen, et al. One characteristic of centrifugal blood pumps is that they are "non-occlusive," which means that when connected to a conduit, the mechanical structure of the pump allows blood to flow in the direction opposite the blood pumping direction. As a result, even though a non-occlusive blood pump may be turned on and operating, blood may actually flow backwards through the pump in the wrong direction, which may have adverse consequences for the patient.
As is known, the primary adverse effect of such backflow is that air may be drawn into the blood within the pumping system and pumped into the patient when the blood flow is later reversed to the proper direction. If the perfusionist or doctor detects that air has been drawn into the pumping system, there are procedures to prevent the air from being pumped into the patient. However, these procedures are tedious and/or time-consuming. Moreover, the perfusionist or doctor may not notice that air has been drawn into the system.
Backflow occurs when the output pressure of the blood pump is exceeded by the load pressure of the patient. This load pressure depends on such factors as the resistance of the arterial and venous systems of the patient, which may vary with the physical size and age of the patient; the resistance of the external pumping system; the relative elevation of the patient with respect to the blood pumping system; etc. If for any reason the load pressure should happen to exceed the output pressure of the pump, backflow will occur.
One blood pumping system that generates a warning that backflow is taking place is disclosed in U.S. Pat. No. 4,778,445 to Hubbard, et al. In that system, the blood flow from a centrifugal pump into the patient is sensed by a non-directional flow sensor. The speed, in RPMs, of a motor connected to drive the pump is also sensed. Generally, the Hubbard, et al. system determines when there is backflow by comparing the change in the actual flow over time with the change in the motor speed over time. For example, if the magnitude of the blood flow to the patient increases in response to an increase in the motor RPM rate, the blood flow is assumed to be into the patient. However, if the magnitude of the blood flow decreases in response to an increase in the motor RPM rate, it is assumed that there is backflow, and an appropriate warning is generated.